Automatic rearview mirrors which automatically control the glare from the headlights of, following vehicles, or when driving away from the setting sun, have been produced and installed in vehicles for many years. Glare reflected in these mirrors has been adjusted by employing motorized prismatic mirrors, liquid crystal shutters and, most successfully, electrochromic mirror reflective elements in which the reflectivity of the mirror is responsive to an applied voltage. While a variety of light measuring and control systems have been proposed and used, such as described in U.S. Pat. No. 3,601,614 issued to Platzer, Jr. and U.S. Pat. No. 3,600,951 issued to Jordan et al., among others, a particularly successful commercial system has relied on two cadmium sulfide light sensors, one sensing ambient light levels and the other sensing rearward glare sources. Typical control systems utilizing this type of devices are described in commonly assigned U.S. Pat. No. 5,715,093 issued to Schierbeek et al.
Many of the characteristics of cadmium sulfide light sensors are well suited to the functional objectives of an automatic mirror control circuit, and their use has contributed to the cost effectiveness of the mirror system in which they are used and the consequent commercial success of these systems.
In recent years, efforts have been made to eliminate cadmium from vehicle systems. In one such effort undertaken in Europe, the vehicle is designed to be recycled, and material, such as cadmium, is restricted. Consequently, it is desirable to utilize light sensors in automatic rearview mirror control circuits which are based upon alternative materials and ideally which achieve the response in performance and cost previously achieved with circuits utilizing cadmium sulfide devices. In this manner, the manufacturer can continue to offer the comfort and advantages of glare control mirrors to the driving public at affordable prices.
Attempts have been made in the art involving vehicle rearview mirror systems having tandem light sensors and light signals that are integrated over predetermined integration periods, Examples of such art include U.S. Pat. Nos. 6,008,486; 6,359,274; 6,379,013 and 6,402,328, the disclosures of which are hereby incorporated herein by reference.
It is also known to provide a display through a mirrored electrochromic cell of an electrochromic mirror, while blocking the view of the display structure or device through the mirrored surface. When such a display is implemented in an interior rearview mirror assembly of a vehicle, it is possible to provide the driver of the vehicle with the full use of the mirror surface when the data display is not required or activated. This also allows the use of a larger display area, and consequently, a larger character size, than is typically possible when the display is located in the mirror frame or bezel, or if a permanent non-mirrored display window is provided within the mirror area. Such a display is commonly referred to as “display on demand”.
Although a display on demand provides the above benefits to a driver of the vehicle, such a display requires brightness or intensity control of the display for optimum readability in all lighting conditions. Traditional rearview mirror displays have a relatively constant brightness background field on which characters are displayed, such as a dark lens surface with low reflectivity or the like. In such displays, it is typical to control the display brightness according only to ambient lighting conditions, such that in bright ambient lighting conditions, the display is bright enough to read, but in low ambient lighting conditions, the display is not so bright that it is annoying or distracting to the driver of the vehicle. For example, a very bright display in dark driving conditions can reduce the driver's ability to discern detail in the forward view, since such a display may cause the driver's pupils to adjust in order to accommodate the bright light source. However, because the reflectivity of a reflective element of an electrochromic mirror is variable or adjustable, the intensity of the display may be further controlled or adjusted to maintain a desired contrast ratio between the display and the reflected scene,